Effects of Chromium Content on the Strength and Toughness of a New, Lower Cost, High Toughness Ultra-high Strength Steel
Abstract
Abstract The ultimate objective of the proposed work is the development of a new, lower cost, ultra-high strength steel of high fracture toughness, which would be a cost effective choice for use in components requiring high strength and high toughness. For example, the new steel could be used to replace the much more highly alloyed steel AerMet100 in Navy landing gear. The new steel is a secondary hardening steel using nickel instead of cobalt to enhance secondary hardening. Nickel has been used because cobalt raises the ductile-to-brittle transition temperature (DBTT) while nickel lowers the DBTT and because nickel is less expensive than cobalt. The composition of the new steel in wt. % is 0.4C-4.5Cr-2Mo-0.5W-0.5V-3to5Ni. Ultra-high strength steels of high fracture toughness currently available are secondary hardening steels such as AerMet100 and the maraging steels. The composition in wt. % of AerMet100 is 0.23C-13.5Co-11Ni-3.2Cr-1.2Mo and that of C250 maraging steel is 0.03C-18Ni-9Co-4.8Mo-0.4Ti-0.1Al. In terms of alloy content the new steel is of significantly lower cost than these steels. The new steel currently has a fracture toughness greater than those of AerMet100 and C250 maraging steel. However, the new steel does not have sufficient strength for Navy landing gear applications. Thus one objective of the proposed research is to increase the strength of the new steel while maintaining high fracture toughness. While the new steel is less expensive than currently available ultra-high strength steels in terms of alloy content, the cost of using a steel also includes the cost of heat treatment. Thus the second objective of the proposed work is to care out studies designed to simplify the heat treatment of the new steel to reduce heat treatment cost. The approach to increasing the strength of the new steel while maintaining high fracture toughness is to increase the chromium content. Work of Pickering and others show that increasing the chromium content should increase the strength of the new steel sufficiently for use in Navy landing gear. Work by Garrison has shown that increasing the chromium content in steels similar to the new steel can also increase the Charpy energy. Thus data suggest increasing chromium content can increase strength while maintaining high fracture toughness. It is proposed to first determine the effects of chromium content on the tensile properties, the room temperature fracture toughness and the ductile-to-brittle transition temperature over a range of tempering temperatures. Then it is proposed to carry out sufficient fractographic and microstructural characterization to understand effects of chromium content on these mechanical properties. The second objective is to simplify the heat treatment of the new steel to reduce the cost of using the new steel. Currently the steel is cooled from the austenitizing temperature, then refrigerated in liquid nitrogen and then triple tempered with refrigeration in liquid nitrogen after each of the three one hour tempers. Initial results indicate that for some tempering temperatures the triple temper can be replaced by a single one hour temper. Studies are proposed to determine the degree of refrigeration actually needed after austenitizing, the degree of refrigeration required after tempering and if triple tempers can be replaced by a single hour temper.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- May 22, 2016
- Source ID
- N000141512055
Entities
People
- Warren Garrison
Organizations
- Massachusetts Institute of Technology
- Office of Naval Research
- United States Navy